In wavelength division multiplexed (WDM) optical communication systems, a single fibre optic cable carries a plurality of optical signal channels, each channel being assigned a particular carrier wavelength. The signal channels are generated using wavelength specific lasers. These channels are then coupled to the traffic fibre using an optical combiner or multiplexer and sent to the next node in the system, possibly via a number of optical amplifiers. At the receiving node, the different wavelengths are filtered out in a demultiplexer and are sent to a respective receiver where they are converted to electrical signals and relayed to further systems or networks.
In the demultiplexer, individual optical channels must be selected from the multiplexed optical signal. To ensure that an optical signal is properly selected, the carrier wavelength launched by the laser transmitter must accurately match the wavelength selected in the demultiplexer. Although the lasers are generally very stable in terms of wavelength, erroneous operation can lead to wavelength drift in an individual channel over time. Similarly, the wavelengths of the demultiplexer passbands can drift. It is important that this wavelength drift is detected and corrected before the channel has deteriorated so much that traffic is disturbed in the channel itself as a result of attenuation, and in neighbouring channels by crosstalk. This is particularly important for dense WDM systems wherein the wavelength spacing between channels is very small, often of the order of a nanometer.
A technique for dynamically stabilising a wavelength selective element in a WDM system is described in U.S. Pat. No. 5,673,129. This document describes that a wavelength reference is used to stabilise the output wavelength of a transmission laser while the reflection wavelength of a Bragg grating used as a wavelength selective element at the receiving end of the system is dynamically adjusted to obtain the maximum reflected optical signal and so accurately correlate the Bragg grating to the corresponding transmitted wavelength. The adjustment of the Bragg grating reflection wavelength is obtained by temperature tuning or adjustment of the amount of physical tension applied to the Bragg grating.
While this arrangement ensures the accurate correlation of the wavelength selective element to the transmitted wavelength, an extreme drift in wavelength in an individual laser will not be corrected and could ultimately lead to crosstalk between adjacent channels. Furthermore, the need for a wavelength reference for each laser necessitates the provision of a relatively large number of potentially costly components.
In an alternative embodiment described in the same patent, the temperature of the Bragg grating is held constant and the signal reflected at the grating is fed back to the transmitting laser and used to dynamically tune the laser to the reflected wavelength by adjustment of the laser temperature. However, the provision of a stable temperature environment for the Bragg grating without recourse to adjustment on the basis of variations in the reflected wavelength is difficult to implement and is dependent on the reliability of normally electrical heating and cooling elements.
In the light of the disadvantages associated with the prior art it is an object of the present invention to provide an arrangement and procedure for controlling the wavelengths of channels in an optical WDM system which is reliable, simple and inexpensive in its implementation.